The present invention relates generally to fiber dispersions used in papermaking and to wet-laid inorganic fibrous sheet material. More particularly, it is concerned with a new and improved process for continuously producing uniform fiber dispersions and with light weight fibrous glass webs of uniform fiber distribution made on production size papermaking machines.
Inorganic fibrous web materials, such as glass fiber papers, have been manufactured for a considerable period of time but have constantly presented the papermaker with special uniform fiber distribution problems. The same has been true for fibrous web material formed predominantly of nonglass synthetic fibers of long length. In this connection, the art has recognized that uniformity of fiber dispersion prior to sheet formation is inexorably tied to uniform fiber formation within the resultant web material. Due to the difficulties associated with achieving the necessary uniform fiber suspension, the resultant inorganic webs of fine diameter fibers were of a heavy basis weight; i.e., about 50 grams/square meter and heavier, since the heavier weight materials were sufficiently thick to mask the non-uniform characteristics of the resultant fiber array. In the typical wet-laid papermaking process, the inorganic fibers have diameters of only a few microns and, as with the nonglass synthetic fibers, are supplied to the dispersing medium in the form of bundles chopped from continuous multiple filament strands. The dispersing medium for glass fibers is usually an acidic aqueous solution and may be slightly viscous in order to promote and maintain the dispersion and isolation of the individual fibers within the multiple filament bundles. Typically, the fibers are placed within the dispersing medium and are agitated within a beater or pulper to effect bundle separation whereupon the stock is conveyed to holding tanks containing conventional mixing units to maintain the fibers within their desired suspended or dispersed condition. As can be appreciated, failure to provide sufficient agitation during the initial dispersion of the fibers causes incomplete separation of the individual fibers and fiber bundles are visible within the resultant continuous sheet material.
In recent years, glass and low denier nonglass synthetic fibers longer than conventional papermaking length; namely, fibers having a length of between about 1/4 inch to one inch and more have been used. However, when these fibers have been dispersed in accordance with the prior known technique, it was found that the individual fibers tended to cling together and snag within the beater and holding tanks and could not easily be redispersed, resulting in clumps or other irregularities within the sheet product. It was also found that the long glass fibers reaccumulated in such a manner as to form multi-fiber groupings exhibiting the configuration of a haystack or spider. Although these "haystacks" can be tolerated in the heavy weight materials and for certain applications where the aesthetic appearance of the sheet material is not of concern, they are considered major defects in light weight materials and for those applications where the glass sheet provides a surface veil or is intended to provide a smooth surface of a reinforced plastic structure.
The thicker, heavy weight glass sheets have been used in vinyl flooring tile and the like to provide dimensional stability. However, the heavy weight glass material has poor resin penetration characteristics and, therefore, poor lamination, resulting in a tendency of the tiles to delaminate. Thin, light weight, hand sheets having good fiber distribution can be individually formed when appropriate care is taken. However, the uniform fiber distribution necessary to provide for elimination of the visually perceptible, overall density variation referred to as the "cloud effect," coupled with substantial minimization of isolated multi-fiber defects or "haystacks," has not been achieved on continuous papermaking machines when producing light weight glass fiber web material.
In a continuous papermaking operation on a production basis, long fiber sheet material is typically produced from very dilute fiber suspensions using an inclined wire or similar type of papermaking machine. In such machinery, there is used a conventional open type headbox of sufficient volume to establish a calm and relatively placid fluid approach to the web forming zone. The advantage of such a headbox is that sufficient time is provided in the headbox for the release of air bubbles from the fiber suspension prior to web formation. However, the desired calm and placid fluid approach has a distinct disadvantage for long glass fiber suspensions. It has been found that as the air bubbles are released at the headbox, they tend to permit and even encourage the formation of fiber "haystacks". The bubbles carry these multi-fiber groupings to the surface of the web material as it is being formed. This provides not only an unacceptable sheet material from a visual appearance standpoint, but also produces an irregular or roughened surface feel that is readily detected by simply passing a hand across the surface of the sheet material.
Accordingly, it is a primary object of the present invention to provide a new and improved process for continuously producing a uniform and homogeneous dispersion of long fibers well suited to the formation of essentially defect-free wet-laid fibrous web material.
Another object of the present invention is to provide a new and improved process of the type described that provides for rapid dispersion of long man-made fibers within a region or zone of high intensity turbulence. Included in this object is the provision for maintaining such a turbulent zone while passing the fibers therethrough for accelerated dispersion.
Still another object of the present invention is to provide a new and improved process of the type described that facilitates rapid and complete dispersion of very long fibers in a continuous flow-through operation by the use of a nonstapling mixing impeller that generates a zone or region of reduced pressure coupled with high intensity turbulence. Included in this object is the provision for a process applicable to both inorganic and organic fibers of long length.
It is a further object of the present invention to provide a new and improved long fiber glass web material of extremely light weight yet of uniform fiber formation that is produced on production size papermaking machinery.
Yet another object of the present invention is to provide a new and improved glass fiber web material of the type described that exhibits a visually perceptive, overall uniform fiber distribution and a minimum of isolated multi-fiber defects. Included in this object is the provision for a light weight glass sheet material of continuous length that is essentially devoid of visible "cloud effect" fiber density variations.
Still another object of the present invention is to provide a light weight glass fiber material that exhibits improved aesthetic and physical properties and renders the material well suited for use in reinforced plastic films, tiles and the like.
Other objects will be in part obvious and in part pointed out more in detail hereinafter.
These and related objects will be achieved in accordance with the present invention by providing a continuous process of producing a uniform fiber dispersion for wet papermaking operations from bundles of long fibers. The process comprises the steps of: (1) providing an initial fiber slurry consisting essentially of a dispersing liquid having a viscosity of at least about 2 cps and long fibers in the form of at least partially unopened fiber bundles, the fibers in said bundles having a fiber length of 1/4 inch and more and a length to diameter ratio of about 400 to 3000; (2) continuously flowing said fiber slurry through an in-line dispersing chamber provided with a plurality of nonstapling impellers having an impeller size relative to the capacity of the chamber of at least 0.1 in./gal., said impellers being adapted for generating trailing regions of reduced pressure and flow disruptive turbulence of high intensity, said slurry being fed continuously through said chamber at a throughput rate sufficiently faster than conventional papermaking fiber dispersing chambers to provide a chamber dwell time of only about ten minutes and less and a dispersion factor greater than 0.005, said factor being the quotient of said impeller size and the throughput rate of said slurry in tons per day; (3) subjecting said slurry to said regions with said turbulence being of sufficient intensity to rapidly open the fiber bundles and disperse the individual fibers during said dwell time within said chamber; and (4) removing the dispersed fibers and liquid from the chamber as a substantially uniform and homogeneous fiber dispersion for subsequent web formation in a wet papermaking operation. The objects are further achieved by providing a light weight inorganic fiber web material comprised of micron diameter inorganic fibers having a fiber length of about 1/4 inch or more and a minor amount of a binder for the inorganic fibers. The web material has a basis weight of about 5-30 grams/square meter, a microvariation in basis weight of less than 10%, a macrovariation in basis weight of less than 5% and an isolated multi-fiber defect count of less than 10 per 100 square feet wherein each defect is an agglomeration of fibers causing a local difference in web thickness of 0.5 mils and more. Further, the web exhibits a visually perceptible overall uniform fiber distribution essentially devoid of "cloud effect" fiber density variations.
A better understanding of this invention will be obtained from the following description and the accompanying drawing of the process including the several steps and the relation of one or more of such steps with respect to each of the others and the article of manufacture possessing the features, characteristics, compositions, properties and relation of elements described and exemplified herein.